A milk tank for storing milk is desired to have a cooling function in order to keep milk fresh. A milk tank with a cooling function is disclosed in the Japanese Patent Laid Open Gazette Hei. 7-8127 for example.
The milk tank described in the gazette comprises a tank body 10′, a water tank (ice bank) 34′ and a refrigerator 30′, which are connected by a pipe 21′ or the like as shown in FIG. 5. In other words, an evaporator 34a′ which allows the refrigerant of the refrigerator 30′ pass through is placed inside the water tank 34′ where the cooling water is cooled to nearly 0.degree. C., and the cooling water is circulated through a cooling jacket 14′ which is provided on the external peripheral surface of the tank body 10′ by means of a pump 23′. The cooling jacket 14′ has an outer wall spaced outwardly from the wall of the tank body 10′ by 10 mm and a large number of alternately arranged dimples 14a′ (projecting parts) which project inwardly from the outer wall to reach to the wall of the tank body 10′. The cooling water supplied from the water tank 34′ through the feed pipe 21′ flows windingly between the dimples 14a′ in the above mentioned space in the cooling jacket 14′ on the tank body 10′ and runs back to the water tank 34′ through the return pipe 22′.
The conventional milk tank represented by the example of FIG. 5 has room for improvement at the following points a), b):    a) A large-sized water tank, i.e., a container for cooling the water which is sent toward the tank body, should be needed. So it is hard to carry both a large-sized water tank, such as the water tank 34′, and the tank body simultaneously, and actually it is difficult to compose a milk tank to be mounted on a truck or the like for transportation. The enlargement of the water tank is caused by the use of cooling water. In order to supply the cooling water of close to 0.degree. C. toward the tank body, the strong cooling must be done by the refrigerator so that ice may adhere to the outside of the evaporator in the water tank. The ice adhering to an evaporator will easily grow large, so the capacity of the water tank should be large enough to secure the quantity of the cooling water sent toward the tank body.    b) The cooling tank has low cooling efficiency and its capacity to cool milk or the contents of the tank body is tend to be insufficient. That is because the flow of the cooling water can hardly keep even velocity in the cooling jacket on the external peripheral surface of the tank body, and the stagnation often occurs in a wide area in the cooling jacket. The difficulty of forming even velocity flow of the cooling water is caused by that the cooling jacket has no guide member therein for the cooling water which flows from the feed pipe to the return pipe, therefore the flow of the cooling water is not regulated. That is, while the cooling water supplied to the cooling jacket from the feed pipe flows so much through areas with lower fluid resistance, it will hardly flow through areas with higher fluid resistance (areas where, for example, the flow takes a longer circuit or longer way). Although the cooling capacity can be improved by increasing the flux of the cooling water or lowering the temperature of the cooling water, the cooling efficiency may not be improved because there still exist the areas where the cooling water cannot flow smoothly in the cooling jacket, and inconveniently the much larger-sized water tank is required.
In addition, the subjects described above are not only about a milk tank but also common to a cooling tank which cools and stores a liquid with a low storage temperature (such as drinks).